The present invention relates to a flat battery used for D.C. power sources of thin devices such as watches, pocket calculators, and memory cards, as well as for memory backup power sources. More specifically the present invention pertains to an ultra-thin battery of especially not greater than 1 mm in thickness, wherein a power generation element is sealed with a case, a sealing plate, and a gasket.
Organic electrolytic solution-containing flat batteries (hereinafter simply referred to as xe2x80x98flat batteriesxe2x80x99), where a power generation element obtained by combining an alkaline metal or the alloy thereof with an organic electrolytic solution is placed in a flat battery case (hereinafter simply referred to as xe2x80x98casexe2x80x99), have high reliability and high potential for size and weight reduction, so that the use of the flat batteries have been increasing as main power sources of various electronic devices and memory backup power sources.
With an increase in demand for card-type thin electronic devices, thinner batteries are required as their main power sources and backup power sources. Typical applications of the card-type electronic devices include ID cards and prepaid cards. Electronic settlement with the IC card will be common in near future, and the market of thin batteries will be expected to expand significantly.
Since the thickness of the typical credit cards is about 1 mm, it is accordingly required that the thickness of the batteries used for the thin devices is not greater than 1 mm.
The following problems, however, arise by simply reducing the thickness of the prior art flat battery to 1 mm or less in order to fulfill the requirement discussed above. In FIG. 13, a sectional view schematically illustrating the structure of a prior art coin type battery is shown. The prior art coin type battery having the thickness of greater than 1 mm as illustrated in FIG. 13 includes a power generation element that is sealed with a case 21 and a sealing plate 22 having a U-shaped curved periphery 22a via a gasket 23.
In the process of manufacturing the battery of approximately 1 mm in thickness, however, there is difficulty in press working to form-the U-shaped curved periphery 22a. Even if press working is allowed, the size is not stabilized and the resulting battery has poor leakage resistance.
Then, as shown in FIG. 14, other flat batteries have been proposed (for example, the U.S. Pat. No. 5,486,431), where by using a sealing plate (hat-shaped sealing plate) 12 including a flat central section projected outward of a case and a flat peripheral section extending substantially parallel to the flat central section, a case 11 is fitted to the periphery of the sealing plate 12 in a case 11 via a gasket 13. FIG. 14 is a sectional view schematically illustrating the structure of this another prior art flat battery.
In the flat battery having the structure of FIG. 14, a power generation element, the gasket 13, and the sealing plate 12 are placed on the case 11 having a top-open circumferential wall, and the opening end of the circumferential wall is curved (bent) inward for sealing. This structure does not require formation of the U-shaped curved periphery 22a on the periphery of the sealing plate 22 as in the case of the coin type battery shown in FIG. 13 to allow press working and relatively facilitate manufacture of the battery.
The gasket 13 is simply in the state of being interposed between the case 11 and the upper face and the lower face of the flat periphery section of the sealing plate 12 and the drawbacks of this structure are accordingly that the force applied to the gasket 13 is insufficient and that the resulting battery has poor leakage resistance. This is because, in the coin type battery shown in FIG. 13, the curved periphery 22a of the sealing plate 22 partly presses the gasket 23 to enhance the leakage resistance when sealing but, in the flat battery shown in FIG. 14, it is difficult to form a curved portion on the sealing plate 12 to partly press the gasket 13.
Further, the case and the sealing plate should be thinned to reduce the thickness of the resulting battery. There is a problem that decreasing the thickness of the case, however, undesirably lowers the strength of the opening end of the case bent inward. This results in lowering the leakage resistance and durability of the battery.
In order to overcome the drawbacks discussed above, the object of the present invention is to improve the shape of the sealing plate and/or the case and thereby to provide a flat battery having excellent leakage resistance and durability and good mass productivity while being as thick as not greater than 1 mm.
For solving the above-mentioned problem in a flat battery constituted by sealing a power generation element with: a case that works as one electrode terminal; a sealing plate that works as the other electrode terminal and has a flat central section projected outward and a flat peripheral section extending substantially parallel to the flat central section; a gasket in the form of film that insulates the case from the sealing plate, the present invention is characterized in that an outer circumferential part of the peripheral section in the sealing plate is bent and/or that the case has a turned edge which fit the peripheral section of the sealing plate fitted via the gasket and said turned edged partly presses the gasket.
In the embodiment wherein the outer circumferential part of the peripheral section of the sealing plate is bent, it is preferable that the outer circumferential part is located in the case and the height of the outer circumferential part is not greater than 95% of thickness of the gasket.
It is also preferable that the bending angle of the outer circumferential part is in a range of 5 to 45 degrees relative to a direction parallel to a bottom face of the case (that is, relative to a horizontal plane).
In the embodiment wherein the case has the turned edge which fit the peripheral section of the sealing plate via the gasket and the turned edge partly presses the gasket, it is preferable that the turned edge has a circular recess partly pressing the gasket.
It is also preferable that the turned edge further has an outer circumferential end bent toward the gasket to partly press the gasket.
It is further preferable that the turned edge is inclined to the bottom face of the case to partly press the gasket. In this case, the angle of the inclination is preferably in a range of 5 to 20 degrees.
In the present invention, it is preferable that the gasket is composed of polyphenylene sulfide, and the partly pressed part of the gasket is pressed at a rate of 10 to 50% of original thickness of the gasket in the non-pressed state.
It is also preferable that the gasket is composed of polypropylene, and the partly pressed part of the gasket is pressed at a rate of 30 to 80% of original thickness of the gasket in the non-pressed state.
It is also preferable that the gasket is composed of polyethylene terephthalate, and the partly pressed part of the gasket is pressed at a rate of 10 to 70% of original thickness of the gasket in the non-pressed state.
It is also preferable that the gasket is composed of polyethylene naphthalate, and the partly pressed part of the gasket is pressed at a rate of 10 to 70% of original thickness of the gasket in the non-pressed state.
It is preferable that the flat battery of the present invention includes the sealing plate of not greater than 0.2 mm in thickness and has a thickness of not greater than 1.0 mm.
The present invention is also directed to a method of manufacturing a flat battery wherein a gasket of a substantially L-shaped cross section and a sealing plate that has a flat central section projected outward and a flat peripheral section extending substantially parallel to the flat central section are placed inside a top-open battery case, and a periphery of the case and the peripheral section of the sealing plate are fastened to each other via the gasket, the method comprising the steps of: curving an open end of the case inward to make the periphery of the case U-shaped and thereby form a turned edge; and recessing at least a portion of the turned edge or bending an outer circumferential part of the peripheral section toward the gasket, so as to form a part at which the sealing plate partly presses the gasket.
The present invention also provides a method of manufacturing a flat battery wherein a gasket of a substantially L-shaped cross section and a sealing plate that has a flat central section projected outward and a flat peripheral section extending substantially parallel to the flat central section are placed inside a top-open battery case, and a periphery of the case and the peripheral section of the sealing plate are fastened to each other via the gasket, the method comprising the steps of: curving an open end of the case inward to make the periphery of the case U-shaped and thereby form a turned edge that is substantially parallel to a bottom face of the case; and inclining the turned edge to the bottom face of the case, so as to partly press the gasket.
In this case, it is preferable that angle of the inclination of the turned edge is in a range of 5 to 20 degrees relative to a direction parallel to the bottom face of the case.